1. Field of the Invention
The present invention relates to a hetero-junction bipolar transistor, especially a double hetero-junction bipolar transistor.
2. Description of the Prior Art
The hetero-junction bipolar transistor (hereinafter denoted by HBT) is going to be commercially realized in devices made of group III-V compound semiconductors and silicon-germanium due to its ability to operate at high speed compared to homo-junction transistors. In the HBT, the minority carrier injection from the base layer to the emitter layer is reduced by the wider band gap energy of the emitter layer than that of the base layer, thereby increasing the current multiplication factor.
When the collector layer has a band gap energy comparable to that of the base layer, the breakdown voltage between the collector layer and the base layer decreases because the electron-hole pair generated by the impact ionization at the collector layer increases. One solution to the degradation of the breakdown voltage is proposed to widen the band gap energy of the collector layer compared to the base layer, which is called as the double hetero-junction bipolar transistor (hereinafter denoted by DHBT). In the DHBT, the collector layer is made of semiconductor material, whose band gap energy is greater than that of the base layer, which suppresses the abrupt increase of the electron-hole pair by the impact ionization at the collector layer and the degradation of the breakdown voltage.
However, a large band gap difference between the collector layer and the base layer in the DHBT leads a spike in the conduction band. This spike functions as a large barrier for conduction electrons, which degrades the operational speed of the transistor and the ability to operate at lower bias condition. Japanese Patent laid open 05-036759 discloses to insert additional layer between the base layer and the collector layer such that the spike in the conduction band disappears. This additional layer has a multi-quantum well structure in which the conduction band continuously varies from the base layer to the collector layer so as not to form the spike. Since the conduction electron can pass the barrier layer of the multi-quantum well by the tunneling, the barrier layer does not regard as an obstacle for the conduction electron.
Another prior art, Applied Physics Letters vol. 59(21), pp. 2697 (1991), discloses the intermediate layer between the base layer and the collector layer as a graded layer, whose band gap energy varies in stepwise configuration.
However, the tunneling probability expected in the former prior art depends on the thickness of the barrier and the depth in the energy band diagram of the well layer. To pass the conduction electron therethrough is to thin the width of the barrier layer. On the other hand, the energy band diagram of the multi-quantum well structure is determined by the width and the depth of the well layer thereof. When the energy band diagram necessary for the intermediate layer between the base layer and the collector layer is to be realized by the quantum well structure, a consistent condition would be hard to obtain. In particular, when the depth of the well layer is first determined by the condition required for the intermediate layer, the number of the well layer must be huge.
When the graded layer is provided between the base layer and collector layer as in the latter prior art and the graded layer is doped, the region between the base layer and the collector layer is not depleted and residual carriers are accumulated therein. This residual carrier brings the increase of the junction capacitance, thereby degrading the high-frequency performance of the transistor.